1. Field of the Invention
The present invention related to video technologies on a computer such as a personal computer and in particular to a video data transfer control system and method allowing video data to be rendered on a display in overlaying fashion.
2. Description of the Related Art
There have been proposed video data transfer controllers having a scrambling/descrambling function to protect the video data from unauthorized copying. More specifically, video data is scrambled by a scrambler and the scrambled video data is transferred through a general-purpose bus. The scrambled video data is descrambled by a descrambler and the original video data is output to a graphics device.
For example, Japanese Patent Application Unexamined Publication No. 6-169307 discloses an encoding/decoding device that changes randomly or at regular intervals an encryption key for encrypting a scrambling key. The video data is scrambled using the scrambling key and the scrambling key is encrypted using the encryption key that changes randomly or at regular intervals. The scrambled video data is transferred through a data bus. The encrypted scrambling key is also transferred and decrypted to produce the scrambling key, which is used to descramble the scrambled video data.
Japanese Patent Application Unexamined Publication No. 9-130733 discloses a data reproducing device that allows a scrambling key to be changed at the timing of transfer instruction of a series of scrambled data. More specifically, the scrambling key is generated by a random data generator when a drive controller has received a read-out instruction from a host computer. It should be noted that a series of data to be transferred to a decoder is stored in a memory of the host computer and thereby there is a time lag between data generation and data decoding. This means that the scrambling key cannot always be changed. Accordingly, the timing of changing the scrambling key is restricted to when a transfer instruction of the series of data is detected. As the result, a new scrambling key has been transferred to the decoder before the series of scrambled data is transferred to the decoder through the memory of the host computer.
In the above prior arts, however, it is not sufficient to protect the video data from unauthorized copying. In the case of the encoding/decoding device disclosed in Japanese Patent Application Unexamined Publication No. 6-169307, when the encryption key has been known by a malicious user, it is easy to copy the video data. Even in the case where the scrambling key itself is changed as disclosed in Japanese Patent Application Unexamined Publication No. 9-130733, the timing of changing the scrambling key is restricted to when a transfer instruction of the series of data is detected and therefore it is not so difficult to descramble the scrambled video data.